Since the end of the 20th century the cellular telephone industry has had enormous development in the world. From the initial analog systems, such as those defined by the standards AMPS (Advanced Mobile Phone System) and NMT (Nordic Mobile Telephone), the development has during recent years been almost exclusively focused on standards for digital solutions for cellular radio network systems, such as D-AMPS (e.g., as specified in EIA/TIA-IS-54-B and IS-136) and GSM (Global System for Mobile Communications). Different digital transmission schemes are used in different systems, e.g. Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA). Currently, the cellular technology is entering the so called 3rd generation, providing several advantages over the former, 2nd generation, digital systems referred to above. Among those advantages an increased bandwidth will be provided, allowing effective communication of more complex data. The 3rd generation of mobile systems is referred to as the UMTS (Universal Mobile Telecommunications System) in Europe and CDMA2000 in the USA. Moreover, it is believed that the first generation of Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc., will be provided by, for example, cellular carriers using the next generation digital cellular system infrastructure.
One evolution in cellular communication services involves the adoption of additional frequency bands for use in handling mobile communications, e.g., for Personal Communication Services (PCS) services. Taking the U.S. as an example, the Cellular hyperband is assigned two frequency bands (commonly referred to as the A frequency band and the B frequency band) for carrying and controlling communications in the 800 MHz region. The PCS hyperband, on the other hand, is specified in the United States to include six different frequency bands (A, B, C, D, E and F) in the 1900 MHz region. Thus, eight frequency bands are now available in any given service area of the U.S. to facilitate communication services. Certain standards have been approved for the PCS hyperband (e.g., PCS1900 (J-STD-007)), while others have been approved for the Cellular hyperband (e.g., D-AMPS (IS-136)). Other frequency bands in which these devices will be operating include GPS (operating in the 1.5 GHz range) and UMTS (operating in the 2.0 GHz range). Each one of the frequency bands specified for the Cellular and PCS hyperbands is allocated a plurality of traffic channels and at least one access or control channel. The control channel is used to control or supervise the operation of mobile stations by means of information transmitted to and received from the mobile stations. Such information may include incoming call signals, outgoing call signals, page signals, page response signals, location registration signals, voice channel assignments, maintenance instructions, hand-off, and cell selection or reselection instructions as a mobile station travels out of the radio coverage of one cell and into the radio coverage of another cell. The control and voice channels may operate using either analog modulation or digital modulation.
The signals transmitted by a base station in the downlink over the traffic and control channels are received by mobile or portable terminals, each of which has at least one antenna. Historically, portable terminals have employed a number of different types of antennas to receive and transmit signals over the air interface. For example, monopole antennas mounted perpendicularly to a conducting surface have been found to provide good radiation characteristics, desirable drive point impedances and relatively simple construction. Monopole antennas can be created in various physical forms. For example, rod or whip antennas have frequently been used in conjunction with portable terminals. For high frequency applications where an antenna's length is to be minimized, another choice is the helical antenna. In addition, mobile terminal manufacturers encounter a constant demand for smaller and smaller terminals. This demand for miniaturization is combined with desire for additional functionality such as having the ability to use the terminal at different frequency bands and different cellular systems.
In the known prior art, it has been commercially desirable to offer portable terminals which are capable of operating in widely different frequency bands, e.g., bands located in the 800 MHz, 900 MHz, 1500 MHz, 1800 MHz, 1900 MHz, 2.0 GHz and 2.45 GHz regions. However, in a near future it is expected that it will be increasingly desirable to offer portable terminals which are also capable of operating in frequency bands located within the range from 3.1 GHz up to and including 10.6 GHz, commonly referred to as the Ultra-Wideband (UWB).
Today, the concept of built-in antennas is well known and commonly used by mobile telephone manufacturers, e.g. SONY ERICSSON® and NOKIA®. However, the performance is still a problem when even wider band capabilities are desirable, e.g. when UWB frequency bands are to be covered. Consequently, in the future prior art antenna designs will still be a limiting factor when developing radio terminals with adequate bandwidth to cover plural bands. A more general problem with built-in antenna is not only small bandwidth, but also significantly worse gain performance than a traditional external antenna, e.g. some kind of stub antenna.
Hence, there appears to be a need for providing a multi-band radio antenna device that mitigates, alleviates or eliminates one or more of the above-mentioned deficiencies or disadvantages in the known prior art. More specifically, there appears to be a need for providing a multi-band radio antenna device having a structure suitable for built-in antennas, which at the same time has a wide bandwidth which enables the antenna to be operable at a plurality of frequency bands.